As types of a fixed type constant velocity universal joint, there have been widely known a Birfield type (BJ) and an undercut free type (UJ). FIGS. 18 and 19 illustrate a fixed type constant velocity universal joint of the Birfield type (BJ).
This fixed type constant velocity universal joint includes an outer joint member 3 having an inner surface 1 including a plurality of (six) track grooves 2 formed equiangularly along an axial direction, an inner joint member 6 having an outer surface 4 including a plurality of (six) track grooves 5 formed in pairs with the respective track grooves 2 of the outer joint member 3 equiangularly along the axial direction, a plurality of (six) balls 7 which are interposed between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6 and transmit torque, and a cage 8 which is interposed between the inner surface 1 of the outer joint member 3 and the outer surface 4 of the inner joint member 6 and holds the balls 7. The cage 8 includes a plurality of pockets 9 which are arranged along a circumferential direction and house the balls 7.
A center of curvature O2 of each of the track grooves 5 of the inner joint member 6 and a center of curvature O1 of each of the track grooves 2 of the outer joint member 3 are offset to each other to opposite sides by equal distances F and F in the axial direction with respect to a joint center O. In other words, provision of such an offset is referred to as “track offset type.”
In this way, in the fixed type constant velocity universal joint of the Birfield type (BJ) illustrated in FIGS. 18 and 19, a groove bottom of each of the track grooves 2 and 5 includes a circular-arc portion. Meanwhile, in the undercut free type (UJ), the groove bottom of each of the track grooves 2 and 5 includes a circular-arc portion and a linear portion (straight portion).
In recent years, for the purpose of weight reduction and compactification, there have been proposed fixed type constant velocity universal joints in which the balls 7 as torque transmitting members include eight balls 7 as illustrated in FIGS. 20 and 21. Specifically, the balls are downsized and the number of the balls is set to eight so that reduction of a load capacity of each of the balls is suppressed, to thereby achieve high performance and compactification.
In one of conventional fixed type constant velocity universal joints including eight balls, a center of each guide groove of the outer joint member is offset to a joint opening side with respect to a spherical center of the inner surface, and a center of each guide groove of the inner joint member is offset to a joint depth side with respect to a spherical center of the outer surface respectively toward opposite sides by equal distances in the axial direction. In addition, a ratio of a pitch circle diameter of the balls and a diameter of the balls is set within a predetermined range (Patent Literature 1).
According to Patent Literature 1, it is possible to provide a more compact fixed type constant velocity universal joint for automobiles, in which a temperature rise and a torque loss during operation are low, and a strength, a load capacity, a durability, and an operating angle which are equal to or higher than those of the fixed type constant velocity universal joint including six balls can be secured.
Further, another of conventional fixed type constant velocity universal joints includes track grooves formed into a helical shape or an inclined shape (Patent Literature 2). In this case, the track grooves are formed into the helical shape or the inclined shape, and in addition, the track grooves include track grooves which are adjacent to each other in the circumferential direction and arranged in plane symmetry. With this setting, fatigue and peeling at contact parts are reduced, and durability and a torque transmitting function of the constant velocity universal joint are enhanced.